Collective pulse amplification in burst mode fiber laser amplifiers in gain-managed nonlinearity regime
Ultrafast lasers have diverse applications, ranging from optical metrology and spectroscopy to microscopy. Outside of the research laboratory, the most important application continues to be material processing, particularly precision micromachining. In traditional ultrafast processing, high-energy pulses at low repetition rates are used. The recently introduced ablation-cooled regime achieves greater speeds and material removal efficiencies through the use of moderate-energy pulses at much higher repetition rates. However, higher repetition rates also demand short pulses, and higher speeds require higher average powers, such as sub-50 fs pulses at kilowatt average powers. Such parameters are far beyond the current state of the art, but there appears to be no fundamental reason such performance could not be achieved. One common approach to exploit the ablation-cooled regime is to utilized bursts, or groups of pulses. However, such burst-mode amplifiers have resulted in relatively pulse durations to date. This thesis aims to overcome this limitation by implementing burst-mode operation in another recently discovered regime, namely, that of gain-managed nonlinear amplification. In ultrafast laser material processing, sub-50 femtosecond pulse widths and kW average powers are desirable. However, such lasers have yet to be reported in the literature due to the complexity of such systems. For obtaining sub-50 femtosecond pulses, the pulse evolution needs to be well-engineered. Additionally, kW average powers require active cooling systems. A practical method to decrease the average powers in the laser systems yet achieve the same pulse energies is to amplify bursts of pulses instead of a continuous stream of them. These bursts allow the energy to be more confined in time and the average power to be lower. In the no-burst case, various nonlinear pulse amplification regimes aid the design processes and produce high-quality pulses. One such regime is the gain-managed nonlinearity, observed after shifting the gain spectrum towards longer wavelengths accompanied by nonlinear spectral broadening. This process results in very broad optical spectra and short pulses for the amplified pulses. Obtaining sub-50 fs pulses in the continuous case is often complicated, and the results are hard to reproduce. However, with the gain-managed nonlinearity regime, sub-50 femtosecond pulses can be routinely obtained by correctly choosing the amplifier parameters. Despite that, with the inclusion of bursts, pulse parameters tend to be much worse, mainly due to having non-uniform pulses within a burst. Hence, obtaining sub-50 femtosecond pulses in burst-mode lasers is very challenging, and so far, no such lasers have been reported. Combining this regime with continuously pumped signal burst lasers, we have obtained pulses with 900 nJ energies and sub-50 femtosecond pulse widths with pulse repetition rates as high as 240 MHz and burst repetition rate of 1 MHz. The design process is aided by gaining knowledge regarding the nonlinear amplification process and the gain medium’s response time through mathematical modeling and numerical simulations. Burst-mode amplification in the gain-managed nonlinearity regime leads to new physical effects, whereby the amplification of one pulse depends on the other pulses constituting a burst.